Investigators are putting together a timeline of Friday's shooting, beginning with Adam Lanza's allegedly shooting his mother while she slept before driving to Sandy Hook Elementary School. NBC's Pete Williams reports.

By M. Alex Johnson, NBC News

Updated at 7:34 p.m. ET: Adam Lanza, who authorities say killed 20 children and six women in Connecticut, shot his mother in the head multiple times before heading to Sandy Hook Elementary School, where he fired hundreds of rounds and died with hundreds more at his disposal, police said Sunday.

It was an extraordinary amount of weaponry that Connecticut State Police Lt. Paul Vance described Sunday. In addition to an assault-style rifle and at least two handguns, he also had a shotgun in reserve in the car he drove to the school.

And when he was found, Lanza, 20, still had "hundreds of rounds" of ammunition in multiple magazines, after having already fired hundreds of rounds inside the school, where he killed himself with a gunshot to the head as emergency crews arrived Friday.

An explanation still hasn't emerged for why Lanza killed the 26 people at Sandy Hook, but Connecticut Gov. Dan Malloy opened a window for speculation when he told NBC News on Sunday that it appeared that Lanza attended the school as a youngster. Earlier reports that his mother, Nancy, may have taught there haven't borne out.

"He attended there — that's what I'm led to believe," Malloy said on NBC's "Meet the Press."

The children — 12 girls and eight boys, all of them 6 or 7 years old — were shot as many as 11 times, H. Wayne Carver, the state medical examiner, said Saturday. It appeared that Lanza had enough weapons and ammunition with him to have killed many, many more.

Police were analyzing the weaponry, along with a computer they found at the Lanzas' home, for possible leads on the gunman's motive, NBC News' Pete Williams reported.

Malloy declined to answer whether any documented evidence had been uncovered that Lanza might have been mentally disturbed. At Western Connecticut State University in Danbury, where he enrolled at about 16 in 2008, there was never any indication of trouble, the university said in a statement Sunday.

Investigators resolutely refused to go into detail about the timing of events Friday during official briefings. But investigators told NBC News that Lanza first killed his mother, an avid gun enthusiast, with her own gun and then took multiple weapons with him as he drove to the school in her car.

To bypass security, Lanza smashed in a window, they said. He shot and killed Principal Dawn Hochsprung, 47, and Mary Sherlach, 56, a school psychologist, before proceeding to a classroom, where he found the door locked.

So he moved on to a second classroom, where he killed everyone he found, before doing the same in a third classroom, investigators believe. He then shot himself.

Although he was carrying three weapons, he used only one of them in all of the school killings — a Bushmaster .223-caliber assault-style rifle similar to the one used by the snipers who terrorized the Washington, D.C., area in 2002. It was purchased legally, they said. He used one of the handguns to kill himself.

Authorities haven't said how Nancy Lanza stored the weapons.

Marsha Lanza, Nancy Lanza's sister-in-law and Adam Lanza's aunt, said there was a good reason for a divorced woman who grew up with guns to have them in the house: self-defense.